Location Estimation and Tracking for Passive RFID and Wireless Sensor Networks Using MIMO Systems

ABSTRACT

Systems and methods for location estimation and tracking for passive RFID and wireless sensor networks in accordance with embodiments of the invention are disclosed. In one embodiment, a process for obtaining location information using an RFID reader system includes transmitting a combined interrogation and ranging signal from a plurality of antennas, where the ranging signal is a pseudorandom signal, receiving a backscattered return signal from an RFID tag at one or more receive antennas, extracting an information signal from the return signal and decoding the information signal to obtain RFID tag data, extracting a received ranging signal from the return signal, and estimating a range to the RFID tag based upon correlation between the ranging signal and the received ranging signal.

RELATED APPLICATIONS

The current application is a continuation of U.S. patent applicationSer. No. 15/479,233, filed Apr. 4, 2017, entitled “Location Estimationand Tracking for Passive RFID and Wireless Sensor Networks Using MIMOSystems” to Ramin Sadr, which claims priority to U.S. ProvisionalApplication No. 62/317,631 filed Apr. 4, 2016 entitled “LocationEstimation and Tracking for Passive RFID and Wireless Sensor Networks,”and U.S. Provisional Application No. 62/481,016 filed Apr. 3, 2017 thedisclosures of which are incorporated by reference herein in theirentirety.

FIELD OF THE INVENTION

Systems and methods in accordance with various embodiments of theinvention relate to radio frequency identification (RFID) systems andmore specifically to the ranging and localization of RFID tags usingRFID systems that incorporate multiple transmit antennas.

SUMMARY OF THE INVENTION

Systems and methods for location estimation and tracking for passiveRFID and wireless sensor networks in accordance with embodiments of theinvention are disclosed. In one embodiment, a process for obtaininglocation information using an RFID reader system includes transmitting acombined interrogation and ranging signal from a plurality of antennas,where the ranging signal is a pseudorandom signal, receiving abackscattered return signal from an RFID tag at one or more receiveantennas, extracting an information signal from the return signal anddecoding the information signal to obtain RFID tag data, extracting areceived ranging signal from the return signal, and estimating a rangeto the RFID tag based upon correlation between the ranging signal andthe received ranging signal.

A further embodiment also includes generating an RFID interrogationsignal waveform having a first frequency, generating a ranging waveformhaving a second frequency, where the second frequency is higher than thefirst frequency, combining the RFID interrogation signal and rangingwaveform signal into a combined interrogation and ranging signal,splitting the combined interrogation and ranging signal to a pluralityof transmit paths through a transmit array filter bank and modifying thesignal in each transmit path using at least one transmit weightingfactor, where each transmit weighting factor modifies a characteristicof the signal, and transmitting a first filtered output signal from eachof the transmit paths of the transmit array filter bank using one of aplurality of transmit antennas in a first interrogation round.

Another embodiment also includes adjusting at least one of the at leastone transmit weighting factor based upon the output of the equalizerfilter bank and a plurality of the calculated time-of-arrivals of theplurality of received return signals, and transmitting a second filteredoutput signal from each of the transmit paths of the transmit arrayfilter bank using one of the plurality of transmit antennas in a secondinterrogation round, where the second filtered output signal is modifiedusing the adjusted at least one transmit weighting factor.

In a still further embodiment, adjusting at least one of the at leastone transmit weighting factor includes applying machine learning toincrease the correlation of the ranging signals.

In still another embodiment, adjusting at least one of the at least onetransmit weighting factor includes applying machine learning to increasethe read rate of the RFID tag.

In a yet further embodiment, receiving a backscattered return signal anRFID tag at one or more receive antennas includes receiving a pluralityof backscattered return signals from an RFID tag at a plurality ofantennas, and the method further includes combining the plurality ofreceived return signals using an equalizer filter bank to produce acombined return signal, and the combining further includes modifyingeach received return signal using at least one receive weighting factor,where each receive weighting factor modifies a characteristic of thesignal.

In yet another embodiment, combining the RFID interrogation signal andranging waveform signal into a combined interrogation and ranging signalincludes adding the two signals.

A further embodiment again also includes adjusting a characteristic ofthe transmitted combined interrogation and ranging signal in asubsequent interrogation round to increase read rate and rangingaccuracy.

Another embodiment again also includes calculating the time-of-arrivalof each of the plurality of received return signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a system diagram illustrating an RFID reader system having asingle transmit/receive antenna in accordance with embodiments of theinvention.

FIG. 1B is a system diagram illustrating an RFID reader system havingseparate single transmit antenna and receive antenna in accordance withembodiments of the invention.

FIG. 1C is a system diagram illustrating an RFID reader system havingtwo or more transmit antennas and two or more receive antennas inaccordance with embodiments of the invention.

FIG. 1D is a photo showing an antenna array with nine antennas eachhaving dual polarization that can be utilized in accordance withembodiments of the invention.

FIG. 2 is a system level overview illustrating an RFID reader system andtag and forward and reverse channels in accordance with embodiments ofthe invention.

FIG. 3A is a system diagram illustrating components of an RFID readersystem configured to provide a ranging estimate in accordance withembodiments of the invention.

FIG. 3B conceptually illustrates an interrogation signal combined with aranging signal for determining direction and/or time of arrival of areceived signal from an RFID tag.

FIG. 3C conceptually illustrates the propagation of a signal from amultiple-input multiple-output (MIMO) transmit antenna array to a MIMOreceive antenna array.

FIGS. 4A-4D illustrate graphs showing example interrogation signalsusing FMO and Miller encoding in accordance with embodiments of theinvention.

FIGS. 4E-4G illustrate graphs showing example ranging signals usingdirect sequence spread spectrum in accordance with embodiments of theinvention.

FIG. 5 is a flow chart illustrating a process for collecting compoundchannel characteristics in accordance with embodiments of the invention.FIG. 6 is a flow chart illustrating a process for generating a rangingestimate using compound channel characteristics and a ranging signal inaccordance with embodiments of the invention.

FIG. 7 is a system diagram illustrating components of a MIMO RFID readersystem for generating a ranging estimate in accordance with embodimentsof the invention.

FIG. 7A is a system diagram illustrating an alternative transmit arrayconfiguration in accordance with embodiments of the invention.

FIG. 8 is a flow chart illustrating a process for generating a rangingestimate using a MIMO RFID reader system and a ranging signal combinedwith an interrogation signal in accordance with embodiments of theinvention.

FIG. 9 is a system diagram illustrating components of an RFID readersystem for generating a ranging estimate in accordance with embodimentsof the invention.

DETAILED DESCRIPTION

Turning now to the drawings, location estimation and tracking forpassive RFID and wireless sensor networks in accordance with embodimentsof the invention are disclosed. Radio-frequency identification (RFID)uses electromagnetic fields to retrieve information storedelectronically on tags that are typically attached to objects. PassiveRFID tags represent a distinct class of transmitter, because theyharvest energy from a received interrogation signal and utilizedharvested energy during transmission. As such, passive RFID tags oftenexperience unstable timing. In particular, the symbol timing and phaseof backscattered symbols can vary from one symbol to the next due to theimpact of depletion of charge on timing circuitry within the passiveRFID tag. Accordingly, RFID systems pose challenges (particularly inindoor multipath environments) that are not readily addressed byconventional communication system design techniques. FIGS. 1A, 1B, and1C illustrate RFID reader systems having different architectures thatcan be utilized in various embodiments of the invention. FIG. 1Aillustrates a single antenna RFID reader system that transmits andreceives from the same antenna. FIG. 1B illustrates a two antenna RFIDreader system having one transmit antenna and one receive antenna. FIG.1C illustrates a multiple transmit antenna and/or multiple receiveantenna RFID reader system that transmits from more than one transmitantennas and/or receives return signals at more than one receiveantennas. Systems and methods in accordance with various embodiments ofthe invention can be utilized within any of a variety of additional RFIDsystem architectures including (but not limited to) systems that utilizemultiple transmit antennas and a single receive antenna element and/orsystems that utilize the same array antenna for both transmittinginterrogation signals and receiving signals backscattered by RFID tags.A nine element array of an RFID system that can be used to transmitcombined interrogation and ranging signals in accordance with variousembodiments of the invention is illustrated in FIG. 1D. As can readilybe appreciated, a 3×3 array can be utilized to transmit signals or totransmit signals and receive signals backscattered by RFID tags.

In many embodiments of the invention, a ranging waveform is combinedinto an interrogation signal transmitted to an RFID tag by multipleantennas and recovered from the return signal received by at least oneantenna from the RFID tag. A ranging estimate can be calculated usingthe returned ranging waveform to estimate the total distance traversedby the interrogation signal and return signal and thereby estimate thelocation of the tag. As is discussed further below, any of a variety ofranging signals can be utilized that can be backscattered by an RFIDtag, where the backscattered signal can be utilized to determine pathlength. In many embodiments, the combined interrogation and rangingsignal is transmitted by a plurality of antennas. In severalembodiments, the polarization, amplitude, timing and/or phase of thetransmitted signals transmitted by each antenna can be adjusted tomaximize the power of the ranging signal backscattered by the RFID tag.In certain embodiments, an RFID tag can be repeatedly interrogated andadjustment made to the weightings applied to the transmitted combinedinterrogation and ranging signals to increase the accuracy of theranging estimate determined using the received ranging signalbackscattered by the RFID tag. In a number of embodiments, the RFIDreceiver system includes multiple antennas and the signals received atthe antenna elements can be combined to increase the signal to noiseratio of the received signals. The combination of the received signalscan also involve modification of the polarization, amplitude, phase,and/or delay of the received signals. In a number of embodiments,machine learning is utilized to determine the modifications applied tothe received signals. In many embodiments, the modifications are appliedbased upon the estimated location of the RFID tag that backscattered thesignals. As is discussed further below, any of a variety of techniquescan be utilized to determine weightings for signals transmitted andreceived via multiple antennas as appropriate to the requirements ofspecific applications in accordance with various embodiments of theinvention. Furthermore, the multiple antennas utilized to transmitand/or receive signals can be configured as a conventional antenna arraywith regularly spaced antenna elements and/or as a distributed array inwhich antenna elements are located in arbitrary locations (with knowngeometric relationships).

The accuracy of a ranging estimate generated in accordance with anembodiment of the invention can be affected by distortions of theranging waveform from when it is transmitted to when it returns to theRFID reader. Distortions can arise from frequency and time dependentsystematic characteristics of the transmitter, receiver, and/or taghardware and/or the medium through which the signals propagate.Furthermore, as a passive tag's stored power depletes, its transmittedsignal typically attenuates. In the context of the following discussion,the term “channel” is used to refer to a communication channel (e.g.forward channel or reverse channel) and “compound channel” is used torefer to the transformation of the transmitted signal that occurs basedupon the combined effects of the forward channel, backscattering of thetransmitted signal by the tag hardware, and the reverse channel. FIG. 2conceptually illustrates an RFID reader system configured to provide aranging estimate in accordance with embodiments of the invention and theassociated forward channel from the transmitter to the tag and returnchannel from the tag to the receiver.

In a number of embodiments, an initial round of RFID tag interrogationsis performed using different frequencies and the received signalsobserved. The observed backscattered signal waveforms can be analyzed todetermine a compound channel model for each tag. When an interrogationsignal is combined with a ranging signal, the compound channel model canbe utilized to remove the data modulated onto the interrogation signalby the RFID tag from the received signal backscattered by the RFID tag.The remaining signal is the received ranging signal that can then beutilized to determine the round trip distance between the readerantenna(s) and the RFID tag. In several embodiments, the ranging signalpossesses characteristics that enable the determination of range withmuch greater precision than can be determined based upon group delayfrom transmitting the interrogation signal alone at multiplefrequencies. In various embodiments of the invention, a frequencyranging signal can be any of a variety of signal types including, butnot limited to, direct sequence spread spectrum (DSSS), ultra wideband(UWB), multitone frequency signaling in which tones are placed in nullswithin the interrogation signal, continuous phase modulation (multi-h),continuous multi-frequency signaling and/or any other ranging signalappropriate to the requirements of a specific application. In manyembodiments, multiple ranging signals are transmitted that includedifferent ranging waveforms to provide a diversity of estimates that canbe utilized to refine a ranging estimate. In a number of embodiments,the ranging signals can be provided as inputs to statistical modelsincluding (but not limited to) particle filters that enable theestimation of the most likely location of an RFID tag given the observedsignals backscattered by the RFID tag.

Ranging Using RFID Systems

A system diagram illustrating components of an RFID reader systemconfigured to provide a ranging estimate in accordance with embodimentsof the invention is illustrated in FIG. 3A. The RFID reader systemincludes an RFID waveform generator to generate an interrogation signaland a ranging waveform generator to generate a ranging signal. An adderor combiner combines the two signals for transmission in the forwardchannel to the RFID tag. The backscattered return signal propagatesthrough the return channel where a RFID waveform generator and rangingestimation block recovers the ranging signal. A direction of arrivalestimation block can estimate the direction from which the return signalarrived at the receive antenna. FIG. 3B conceptually illustrates aninterrogation signal combined with a ranging signal for determiningdirection and/or time of arrival of a received signal from an RFID tag.FIG. 3C conceptually illustrates the propagation of a signal from a MIMOtransmit antenna array to a MIMO receive antenna array.

FIGS. 4A-4D illustrate graphs showing example interrogation signalsusing FMO and Miller encoding in accordance with embodiments of theinvention.

FIGS. 4E-4G illustrate graphs showing example ranging signals usingdirect sequence spread spectrum in accordance with embodiments of theinvention.

RFID reader systems in accordance with many embodiments of the inventionmay include multiple transmit antennas and/or receive antennas. Inseveral embodiments, an RFID reader system includes four transmitantennas and four receive antennas. In RFID reader systems that includemultiple transmit antennas and multiple receive antennas, the rangingsignal can be encoded using one or morespace-time-frequency-phase-polarization (STFPP) code. As discussedbelow, space time STFPP codes yield highly correlated signals at acorrect range hypothesis in multiple input multiple output (MIMO)communication systems. Similar to a space-time code, STFPP codes specifyone or more characteristics of a signal (space, time, frequency, phase,and/or polarization) to vary in transmitting multiple copies of thesignal from multiple antennas. Specific STFPP codes and the manner inwhich they can be utilized to recover ranging information in accordancewith a number of embodiments of the invention are discussed furtherbelow.

RFID reader systems in accordance with embodiments of the invention mayutilize a phased antenna array and/or a distributed antenna array suchas the various arrays described in U.S. Pat. No. 8,768,248 entitled“RFID Beam Forming System” to Sadr, the disclosure from which relevantto antenna arrays having multiple elements is hereby incorporated byreference in its entirety.

RFID reader systems in accordance with embodiments of the invention mayutilize distributed antennas such as those described in U.S. Pat. No.8,395,482 entitled “RFID systems using distributed exciter network” toSadr et al., the disclosure from which relevant to distributed antennaarchitectures is hereby incorporated by reference in its entirety.

The reading of RFID tags can involve timing and phase uncertainty in thebackscattered signal returned from a tag. Several RFID reader systems inembodiments of the invention can detect timing and phase uncertaintyusing techniques such as those described in U.S. Pat. No. 7,633,377entitled “RFID Receiver” to Sadr, the disclosure from which relevant todetecting time and phase uncertainty of a backscattered signal is herebyincorporated by reference in its entirety.

RFID tag location may be determined using phase differences andfrequency differences as described in U.S. Pat. No. 8,072,311 entitled“Radio frequency identification tag location estimation and trackingsystem and method” to Sadr et al. and/or using the various statisticalmodelling techniques also disclosed in U.S. Pat. No. 8,072,311, thedisclosure from which relevant to tag location estimation is herebyincorporated by reference in its entirety.

Systems and methods for performing ranging and determining the locationof RFID tags in accordance with various embodiments of the invention arediscussed further below.

Interrogation Phase

In several embodiments, the process of obtaining range information to aparticular RFID tag includes a first phase involving developing acompound channel model for the RFID tag and then a second phaseinvolving using the compound channel model to estimate the round tripdistance from the RFID reader antenna(s) to the RFID tag. In manyembodiments, a first phase is referred to as the inventory phase. In theinventory phase, one or more tags are read multiple times at differentfrequencies using an interrogation signal that may not include a rangingwaveform. Characteristics of the difference between a return signalreceived from the tag(s) and an ideal waveform can be stored as compoundchannel estimates that are indicative of the characteristics of theforward and receive channels and backscattering by the RFID tag itselfIn several embodiments, a return signal is the backscattered signalreceived from the RFID tag. A process 500 for reading one or more tagsin an inventory phase in accordance with several embodiments of theinvention is illustrated in FIG. 5.

The process 500 includes reading (510) one or more RFID tags. Each readincludes an RFID reader system transmitting an interrogation signal toan RFID tag and receiving a backscattered return signal from the RFIDtag. In many embodiments, an RFID tag is read multiple times usingdifferent frequencies. In various embodiments, the RFID reader systemmay utilize a single transmit/receive antenna, a single transmit antennaand a single receive antenna, a single transmit antenna and multiplereceive antennas, multiple transmit antennas and a single receiveantenna, or multiple transmit antennas and multiple receive antennas. Inmany embodiments, the tags store and/or backscatter information inaccordance with the Electronic Product Code (EPC) Gen 2 standardpublished by GS1 AISBL, a non-profit association registered in Belgium,the disclosure of which is hereby incorporated by reference in itsentirety. In other embodiments, information may be stored in any of avariety of formats and/or any of a variety of communication protocolscan be utilized to transmit information to/from the RFID tag asappropriate to a particular application.

The backscattered return signal is received (512) and the code withinthe signal is decoded. An ideal waveform is synthesized (514) for therecovered code. The difference between the ideal waveform and thereceived return signal provides components of characteristics of thecompound channel from the transmitter to the tag to the receiver. Theamplitude, phase, and I and Q component imbalances can be extracted(516) for each tag read at the given frequency. The compound channelestimates for each tag being read over different frequency channels canbe stored in a channel management database or similar storage. Thecompound channel estimates can be used to train machine learningalgorithms in synthesizing a reference waveform that accounts for thecharacteristics of the compound channel and/or to estimate the distanceof the tag from the reader as will be discussed below. Although specificprocesses for generating compound channel estimates in accordance withembodiments of the invention are discussed above with reference to FIG.5, any of a variety of processes may be utilized as appropriate to aparticular application.

Ranging Phase

In several embodiments, a second phase is referred to as the rangingphase. A process 600 for ranging an RFID tag in accordance withembodiments of the invention is illustrated in FIG. 6.

The process 600 includes synthesizing (610 ) a model return signalwaveform for a tag using compound channel estimates so that the waveformcompensates for systematic error in the compound channel. Aninterrogation signal including a ranging waveform can be sent to the tagand the actual backscattered return signal is received (612). Thereceived return signal is compared to the synthesized return signal andthe ranging waveform is recovered (614) from the difference between thesignals. I and Q components of a signal can be replicated from I and Qsamples taken during the interrogation phase. In several embodiments,the synthesized return signal is generated for a particular EPC code(e.g., a specific data payload) that is used for the inventory andranging phases. Comparison of the signals can be made in time (e.g.,using a finite impulse response filter) or frequency (e.g., using a fastFourier transform) domains to recover the ranging waveform. For examplein the frequency domain, a fast Fourier transform (FFT) can be performedwith zero weights and then an inverse FFT performed to cancel out thedata payload of the signal. As can readily be appreciated, the receivedsignal can also be utilized to extract data backscattered by the RFIDtag as a separate process to the extraction of the ranging signal.

A range estimate for the distance from the transmit antenna to the tagto the receive antenna can be generated (616) from the received rangingwaveform. In some embodiments, the range can be determined by measuringthe round-trip time of the signal from transmit to receive. In otherembodiments, the range can be determined from the difference in phase oftwo received signals returned from interrogation signals at twofrequencies. The specific technique utilized is largely dictated by thespecific ranging waveform transmitted by the RFID receiver.

In several embodiments, the range estimates can be provided (618) tomachine learning processes to refine the compound channel estimates.

Although specific processes for determining the range of an RFID tag inaccordance with embodiments of the invention are discussed above withreference to FIG. 6, any of a variety of processes may be utilized asappropriate to a particular application.

Range Estimates using Multiple Antennas

Multiple-input and multiple-output (MIMO) radio architectures utilizemultiple transmit and/or receive antennas to exploit multipathpropagation. Because of the standing wave phenomenon, a transmittedinterrogation signal can have nulls at certain locations and tags atthose locations may not respond. Therefore, repeating transmissions withdifferent characteristics of the interrogation signal such aspolarization and phase and transmitting multiple signals from differenttransmit antennas can be useful in reducing the number of nulllocations. Transmit arrays and distributed exciters that can be used forMIMO interrogation include those described in U.S. Pat. No. 8,395,482 toSadr et al., the disclosure from which relevant to activating RFID tagsusing multiple transmit antennas and constructing RFID receiver systemsutilizing distributed architectures is hereby incorporating by referencein its entirety.

In many embodiments of the invention, an RFID reader system includes Ntransmit antennas and M receive antennas. The antennas can be used formultiple reads of a tag to find the best estimate for the range of thetag. A set of transmit antennas may be configured in an array (e.g., anear-field array) or distributed throughout an area. Similarly a set ofreceive antennas may be configured in an array or distributed throughoutan area.

Space-time code (STC) methods can be used to determine waveforms totransmit at different transmit antennas to provide peak correlation whena range hypothesis correctly estimates the distance of the tag from thevarious antennas. A matrix of waveforms in space-time coding can bedesigned so that elements of the matrix are orthogonal. In severalembodiments, code design can utilize a virtual array of N by M antennas.Each of the M virtual arrays includes a receive antenna and the entireset of N transmit antennas. Algorithms that can be used to designappropriate space-time codes include those discussed in U.S. ProvisionalApplication No. 62/317,631, incorporated by reference above. The processof determining what signal each antenna should transmit at each time tcan be referred to as Algorithm Ranging Code Design. While space-timecodes can provide significant improvements in ranging resolution, any ofa variety of techniques can be utilized to determine the delay between atransmitted and received signal including the use of an array of matchedfilters that each include small time shifts to enable a precisealignment of a transmitted and oversampled received signal that yields amaximum correlation and/or a correlation that satisfies an appropriatethreshold. As can readily be appreciated, the specific techniqueutilized to detect the delay between transmission and receipt of abackscattered signal is largely dependent upon the requirements of agiven application. As is discussed further below, antennas that includemultiple ports enabling transmission using different polarizations alsoenable the adjustment of antenna polarizations to improve the peakreceived power of a received ranging signal backscattered by an RFIDtag. MIMO RFID tag ranging and interrogation techniques in accordancewith various embodiments of the invention are discussed further below.

Background to MIMO Tag Interrogation

A fundamental practical challenge in the application of sensors attachedto objects is the location of the antenna of the sensor with respect tothe reader/interrogator antenna. Specifically, energy harvestingefficiency of passive sensors and RFID tags is highly dependent on theangle of incident wave with respect to the position of the antenna ofthe sensor device. For example if the sensor antenna is 2-dimensionaland the sensor antenna is vertically positioned with respect to overheadreader antenna radiating vertically, there may not be sufficientexposure of a sensor's antenna surface to harvest sufficient energy tocommunicate.

Another challenge is standing wave phenomenon, where at a fixedfrequency and stationary sensor field in a rich scattering environment,electromagnetic waves superimpose constructively and destructively. Inthe latter case, when adding destructively, the flux density radiatedfrom a fixed reader antenna exhibits nulls in the energy density atrandom positions, spaced at fractions of the wavelength within thephysical space in the field of view of radiating antenna. The net effectof this phenomena can also result in passive sensors failing to harvestenough energy turn on and communicate with the reader system.

With respect to sensor/RFID tag antenna positions, polarization mismatchcan also present a further challenge to deliver sufficient energy to thesensor, accentuated further in non-line-of-sight (NLOS). Systems andmethods in accordance with many embodiments of the invention canovercome these limitations to detect and process the payload from thesensor device, but also identify the location of the device.

In many embodiments, both transmitter and receiver antennas of the RFIDreader system are dually polarized. That is either having left and rightpolarization for circular polarization, or else vertical and horizontalpolarization for linear polarization.

In applications where there is NLOS with the sensor and a richscattering environment, classical transmit beamforming with maximizingthe power in the look direction (e.g., using a steering vector) of thearray can fail due to presence of multipath in the forward channel(i.e., from the transmitter to the tag), due to obstructions between theRFID tag and reader transmitting antenna. In some of these cases it canbe desirable to “steer” the beam in such a manner to deliver maximumpower to the tag antenna. In the receive array, it can be desired to addall the signal components (from both polarizations of each antennaelement) constructively, however, in a multipath environment thereceived signal is the superposition of the Cartesian product of allpossible paths from the transmit array to the sensor and from the sensorbackscattered signal to the receive antenna(s).

For addressing these limitations systems and methods in accordance withvarious embodiments of the invention can utilize one or more of thefollowing approaches:

Diversity in: Space (transmit and receive antenna arrays), Time (byquerying the sensor many times), Frequency (by using different frequencychannels for different query rounds and/or different antennas), Phase(by varying the phase for different query rounds and/or differentantennas), and/or Polarization (employing dually polarized antenna)

Ranging signal to make accurate measurements of time difference ofarrival (TDOA) of the signal at the receiver array

“Analysis by synthesis” method to iteratively perturb the phase, and/oramplitude for each polarization (collectively referred to hereinafter asweighting) of each transmit antenna element, observe the underlyingbehavior of the synthesized signal at the receiver, and use adaptivesignal processing and/or machine learning techniques to adapt thetransmit antenna weighting for the next query cycle. Beamformingcoefficients can be used as initial weights.

MIMO Channel and Signal Model

The channel model for dual polarized antennas is typically describedusing four complex coefficients. Quaternions can be used to describe thechannel by a single quaternion number. For example, see B. J. Wysocki,T. A. Wysocki, and J. Seberry, “Modeling Dual Polarization WirelessFading Channels using Quaternions,” IEEE, 1-4244-0368-5/06, 2006, theportions of which relevant to quaternion representation of dualpolarized antennas are hereby incorporated by reference in theirentirety. In general, let x^(H)(t) represent horizontal polarizationwith superscript H and x^(v)(t) the vertical polarization respectively.The relationship between the complex valued channel model and quaternionchannel model is as follows:

Re(h ^(HH))=Re(h ^(VV))=q ₁

Im(h ^(HH))=−Im(h ^(VV))=q ₂

−Re(h ^(VH))=Re(h^(HV))=q ₃

Im(h ^(VH))=Im(h ^(HV))=q ₄

q=q ₁ +q ₂ i+q ₃ j+q ₄ k

where Re( )is the real part and Im( )is the imaginary part, h is complexchannel coefficient (for transmit/receive patch pair), and q representsthe quaternion of channel coefficients.

The above represents the channel impulse response from a horizontal orvertical transmit antenna to a horizontal or vertical antenna. Usingquaternions simplifies the representation of the polarization so thateach patch is represented by a single quaternion as opposed to twoseparate channel impulses.

Assume that the electromagnetic planewave traveling is represented by

${g(t)} = {\begin{pmatrix}{g^{H}(t)} \\{g^{V}(t)}\end{pmatrix} = {\left( \begin{pmatrix}{a^{H}(t)} & e^{j\; {\phi^{H}{(t)}}} \\{a^{V}(t)} & e^{j\; {\phi^{V}{(t)}}}\end{pmatrix} \right){s(t)}}}$

Let θ ∈ [0, 2π] and φ ∈ [0, π/2π be the azimuth and elevation angle ofthe transmitted/received signal and two angles γ ∈ [0, π/2] and η ∈ [0,2π] be the polarization auxiliary angle and polarization phasedifference. The output of the transmit (Tx) array of the i-th element is

x _(i)(t)=a(θ_(i), φ_(i), γ_(i), η_(i), β_(i))S _(i)(t)+n(t)

where a(θ,φ,γ,η,β) is a quaternion valued function and n(t) isquaternion valued additive white Gaussian noise (AWGN).

Systems for Estimating Range Using Multiple Antennas

In many embodiments of the invention, a location estimation RFID readersystem includes a transmitter having at least two transmit antennas anda receiver having at least two receive antennas. A location estimationRFID reader system that may be utilized in accordance with severalembodiments of the invention is illustrated in FIG. 7. In theillustrated embodiment, the transmitter 702 of the reader system 700includes a waveform synthesizer 704, a transmit array filter bank 706,and transmit antennas 708 that can be arranged in one or more transmitantenna arrays #1, #2, . . . L.

In many embodiments, the waveform synthesizer 704 is configured togenerate an interrogation signal waveform. In several embodiments, thewaveform synthesizer 704 generates an interrogation signal as a sensorwaveform in accordance with a standard for RFID systems such as (but notlimited to) the EPC Gen 2 protocol. The interrogation signal may utilizeany of a variety of types of encoding schemes appropriate for tagactivation, such as FM0 or Miller. In further embodiments, the waveformsynthesizer 704 is configured to generate a ranging signal waveform,such as those described further above.

The transmit array filter bank 706 includes two or more transmit signalpaths 710 to which the combined interrogation and ranging signal isdistributed. Each transmit signal path 710 individually processes thecombined interrogation and ranging signal using transmit weightingfactors for each transmit signal path 710. Transmit weighting factorscan include, but are not limited to, frequency, phase, amplitude, and/ordelay. Additional variations can include timing (e.g., which antennastransmit in which interrogation round) and/or polarization (whichpolarization to transmit the signal, e.g., horizontal or vertical, leftor right circular). Transmit weighting factors may be updated forsubsequent interrogation rounds as discussed in greater detail furtherbelow.

Each transmit signal path 710 is connected to a transmit antenna. In theillustrated embodiment, a pair of transmit paths for horizontal andvertical polarization are connected to ports for horizontal and verticalpolarization of a transmit antenna. Similarly, other transmit antennashave pairs of horizontal and vertical transmit paths leading to them. Inother embodiments, a transmit signal path can be connected in differentways to a transmit antenna to direct the polarization of the signal whenit is transmitted from the antenna. In many embodiments, a singlepolarization of each receive antenna is utilized.

The receiver 720 of the reader system 700 includes an equalizer filterbank 722 and a waveform analyzer 724. The equalizer filter bank 722combines the signals received from multiple antennas into a singlesignal. The waveform analyzer 724 can be used to decode the data payloadfrom the signal, estimate location using the embedded ranging signal,and/or perform channel estimation based on an inventory round ofinterrogations. The operation of an RFID receiver that may be utilizedin accordance with various embodiments of the invention is described inU.S. Pat. Nos. 7,633,377 entitled “RFID Receiver” to Sadr and U.S. Pat.No. 8,768,248 entitled “RFID Beam Forming System” to Sadr, the relevantportions of which are hereby incorporated by reference in theirentirety.

Processes for Estimating Range Using Multiple Antennas

In many embodiments of the invention, a process for estimating range toan RFID tag can utilize multiple transmit antennas to send multiplecopies of an interrogation signal that includes an embedded rangingsignal. A process for transmitting multiple interrogation signals todetermine range in accordance with embodiments of the invention isillustrated in FIG. 8. The process 800 includes generating (810) aninterrogation signal waveform. In many embodiments, the interrogationsignal waveform conforms with the EPC Gen 2 standard. In furtherembodiments, the interrogation signal waveform utilizes FM0 encoding,while in still other embodiments it utilizes Miller encoding.

The process includes generating (812) a ranging signal waveform. In manyembodiments, the ranging signal waveform is a pseudo-random signal.Types of signals that can be utilized for a ranging signal in accordancewith various embodiments of the invention include, but are not limitedto, direct sequence spread spectrum (DSSS), ultra wideband (UWB),multitone frequency signaling in which tones are placed in nulls withinthe interrogation signal, continuous phase modulation (multi-h),continuous multi-frequency signaling and/or any other ranging signalappropriate to the requirements of a specific application. In severalembodiments, the ranging signal waveform is optimized by being based ona previous channel measurement of the same tag in a previous querycycle.

The interrogation signal waveform and ranging signal waveform arecombined (812). In many embodiments, the signals are added together. Thecombined interrogation and ranging signal is provided (814) to transmitsignal paths for each polarization of each antenna. In severalembodiments, each antenna has input ports for horizontal and verticalpolarization. One or both may transmit at the same time.

The transmit paths of the filter bank operates on each I and Q signal ofeach element for optimized phase rotation and amplitude weighting usingtransmit weighting factors. Each polarization is induced on eachpolarization rail that can be represented Z(t) by a quaternion at theoutput of each antenna element. As will be discussed in greater detailfurther below, machine learning and other techniques can be used toupdate transmit weighting factors from query round to query round with agoal of maximizing the peak correlation of the transmitted and receivedranging signals. As is discussed further below, a ranging signal with anarrow peak autocorrelation can be utilized. When weightings areappropriately applied, the peak correlation between the received rangingsignal and the transmitted ranging signal is increased and the width ofthe peak can be narrow, enabling accurate range estimation. In a numberof embodiments, beamforming coefficients can be used as initialweighting factors and these weighting factors are adjusted in an effortto increase peak correlation until one or more appropriate stoppingcriteria are satisfied.

The interrogation and ranging signal activates an RFID tag that returnsa backscattered return signal. Backscattered return signals are received(816) at one or more receive antennas. In several embodiments, thereceive array processes a signal, that can be represented as aquaternion, by optimal phase rotation as estimated by simultaneouslyconsidering the transmit weighting factors (e.g., phase and/orpolarization). In further embodiments, measurements are provided forpreequalization of the signals at the transmit array. When there aremore than one receive antennas, the signals received at the antennas canbe combined using receive weighting factors. The weighting factorsutilized to perform the combination can be similar to those outlinedabove with respect to the weightings applied to the transmitted signal.

The correlation is found (818) between the interrogation signalgenerated at the transmitter and the received interrogation signalportion extracted from the received signal and/or between the rangingsignal generated at the transmitter and the received ranging signalportion extracted from the received signal.

In several embodiments, machine learning or other techniques based uponthe radiometric data can be used to update transmit weighting factorsand/or receive weighting factors based upon the received radiometricdata and/or a ground truth of reading tags in known locations. The useof machine learning is discussed further below.

Here, analysis by synthesis is useful to singulate on a specific sensorto de-noise the location measurement for each iteration. When transmitand receive reader antennas are the same (typically referred tomono-static operation). The location algorithm entails solving for thedistance from the tag to the reader. When operating in bi-static mode,where separate receive and transmit antennas are used, the distance oftransmit antenna to the tag and tag to the receive antenna is obtained.RFID reader systems in accordance with embodiments of the invention mayutilize distributed antennas such as those described in U.S. Pat. No.8,395,482 entitled “RFID systems using distributed exciter network” toSadr et al., incorporated by reference above. With multiple transmitantennas in distributed excitation methodology, the location of tagcanbe inferred from distance measurement from each antenna. RFID taglocation may be determined using phase differences and frequencydifferences as described in U.S. Pat. No. 8,072,311 entitled “Radiofrequency identification tag location estimation and tracking system andmethod” to Sadr et al., incorporated by reference above. In manyembodiments, the ranging signal enables much more precise phase and/ordelay estimates than can be obtained by comparisons of the interrogationsignal. Accordingly, embedding the lower amplitude ranging signal withinthe interrogation signal can achieve significant improvement in rangingresolution.

While specific processes are described above with respect to FIG. 8, oneskilled in the art will recognize that any of a variety of processes maybe utilized for ranging estimates in accordance with embodiments of theinvention as appropriate to a particular application. For example, aprocess may utilize one transmit antenna and multiple receive antennas,multiple transmit/receive antennas, or any of a variety of architecturesdiscussed herein.

Received Signal Model

The received signal y(t) at the i-th element can be represented as aquaternion valued signal

$\begin{matrix}{{y_{i}(t)} = {{\sum\limits_{n = 1}^{M}{H_{ni}{x_{n}(t)}}} + {n_{i}(t)}}} & (1)\end{matrix}$

or in matrix form y=Hx+n, where n(t) is assumed to be quaternion valuedadditive white Gaussian noise and H_(ni) is the composite channel modelfrom other transmit antenna to its receive antenna element, accountingfor cascade of forward, backward and transmit and receive array factorand steered response along each polarizations. We drop the index forreceiver element array for notational simplicity. The transmitted signalfrom each antenna port (i.e., horizontal/vertical)

s(t)=A(1+αp(t)+βm(t))cos(2πft+θ)   (2)

where s(t) is related to x(t) in equation (1) with H ∈ Q^(M×N)Let H^(Δ)denote the quaternion conjugate-transpose of H where

H=H ₀ +H ₁ i+H ₂ j+H ₃ k ∈ Q ^(M×N)

where Q denotes the quaternion field. Let Λ represent the equalizationmatrix size of M×N. Λ is chosen such that ΛH^(Δ)HΛ^(Δ)=I (3)

There are numerous time and frequency domain approaches for solving forΛ.

After removing the DC component from equation (2), for the packetdecoder estimates m(t), that is the payload from the sensor andsubtracted from (2). Techniques for estimating a payload from a receivedsignal including (but not limited to) Multi-Symbol Non-Coherent and SoftInput Soft Output decoders are discussed in U.S. Pat. No. 7,633,377entitled “RFID Receiver” and U.S. Pat. No. 8,400,271 entitled “RFIDReceiver,” the relevant portions of which are hereby incorporated byreference in their entirety. Then a correlation is performed inlocalization algorithm for estimating time difference of arrival (TODA),based on the ranging signal.

In the waveform analysis by synthesis block, the following objectivefunctions are used to find optimal Ø,θ,γ,η and the weighting factor βfor each polarization. Objective function is

${\underset{\rho}{Max}}^{{y}^{2}}$

subject to unitary condition in equation (3). Subsequently transmitupdate signal can be used to pre-equalize the transmit array for thenext query round. Transmit control signal is used for directing thetransmit array to proceed with the selected frequency and phasesequences for the next round.

Non-Line of Sight

When the tag is in NLOS (non-line of sight), multiple transmit arrays ina grid configuration can be used and machine learning employed on aquantized grid of 3-dimensional Euclidean space to arrive at the finallocation estimate. A set of sensors/tags with known locations in the 3Dquantized grid can be used to train the machine learning model and adaptthe equalization matrix to match the location of known sensors. Duringthe training period, it is also possible to consider different antennaorientation of the tag with the known location. In which case, thequaternion valued estimated channel response is extended to account theorientation of the tag antenna as input training parameter into themachine learning algorithm. The values obtained at the end of thetraining period are subsequently used for each grid location, as initialcondition of the antenna radiating in that particular gridconfiguration.

Further discussion of the relationship between representation as acomplex value signal with quaternion value signal can be found in J.Tao, W. Chang, “Adaptive Beamforming Based on Complex QuaternionProcesses,” Mathematical Problems in Engineering, Vol. 2014, 2014(available at: http://dx.doi.org/10.1155/2014/291249), the relevantportions of which are hereby incorporated by reference in theirentirety.

Arrays over Quaternions

Noting multiplication with quaternions is non commutative, thedefinition of correlation becomes ambiguous. Discussion of correlationin quaternions can be found in S. Blake, “Perfect Sequences and Arraysover the Unit Quaternions,” 2016, December, unpublished, the relevantportions of which are hereby incorporated by reference in theirentirety. Defining right and left correlation:

θ^(right)(τ)=Σ_(i=0) ^(n−1) s _(i+τ) s _(i) and θ^(left)(τ)=Σ_(i=0)^(n−1) s _(i+τ)s_(i)

A quaternion valued signal is called perfect θ^(left)(τ)=θ^(right)(τ)=0.By adding the constraint in our optimization problem stated in equation(4), we can restrict our choice of transmit pre equalizer over roots ofunity.

Number of perfect sequences constructed exist [Ret]. In general theseconstructions of one form:

$s = {\left\lbrack s_{a,b} \right\rbrack = {i^{\lfloor\frac{f{({a,b})}}{c}\rfloor}j^{\lfloor\frac{g{({a,b})}}{d}\rfloor}}}$

where a, b, is sgr of the array f(a,b), g(a,b) bivariate polynomialswith integer coefficients and c and d positive integers.

Computing Covariance Matrix: Received Signal

We assume one quaternion valued stochastic process of received signal is“centered” that is one probability density function is normallydistributed and takes on the form:

$\left. \underset{\_}{y} \right.\sim{N\left( {0,\Gamma} \right)}$${P\left( \underset{\_}{y} \right)} = {\frac{1}{\sqrt{\pi}{\det (\Gamma)}}e^{{- \frac{1}{2}}g^{t}\Gamma^{- 1}y}}$

The derivation of correlation matrix ┌=R [yy*] is discussed in N. LeBihan, “The geometry of proper quaternion random variables,”(unpublished), the relevant portions of which are hereby incorporated byreference in their entirety.

Machine Learning for MIMO Radar

FIG. 9 is a system diagram illustrating components of an RFID readersystem for generating a ranging estimate using machine learning inaccordance with embodiments of the invention. A sensor field with knownlocation of each sensor in

³ forms the training dataset for the function block in FIG. 9. Duringtraining phase, each dataset is used for computing the error vectore=|x−{circumflex over (x)}|². Number of different approaches can be usedfor training the model, such as support vector machine (SVM), randomforest, deep neural convolutional network to classify the dataset intodiscrete regions. These regions can range to anything from simplicialcubes, triangles, or Veronoi regions. In several embodiments, the groundtruth is determined by placing the tags in a grid and continuinglearning until the solution converges.

Once the training phase is completed, the pre-equalization matrix is setto initialize the query round for the particular antenna and array lookdirection in the specific region labeled in the training phase. In manyembodiments, machine learning utilizes radiometric data from the receivecombiner and equalizer filter bank. Radiometric data can be representedby quaternion c_(i) ∈ Q^(M) for antenna i=1 . . . k. The possiblelocations can be quantized into a three-dimension lattice (ortwo-dimension grid) for each position x₁ ∈

² with i=1, . . . L.

An operator G can be found to map

${\underset{\_}{c}\overset{G}{\rightarrow}{\underset{\_}{v}\mspace{14mu} {if}\mspace{14mu} \hat{x}}} \in \left\{ {{u_{i}:{{d\left( {u,u_{i}} \right)} < {ɛ\mspace{14mu} {where}\mspace{14mu} v_{n}}}} = 1} \right.$

if the tag is in the position, 0 otherwise.

The output of this processing is a hypothesis detector test with asingle non-zero digit in each iteration (e.g., 00010 . . . 0)corresponding to the presence of the tag position in the particularregion. In this case the hamming distance for each respective quotientcan be maximized for training the machine learning algorithm. As canreadily be appreciated, any of a variety of processes can be utilized inaccordance with the requirements of a given application.

Machine Learning Training

In training the neural net/machine learning model, with known locationof each tag in the query, the labeled binary vector v is captured (i.e.,ground truth) and the model is iterated with fixed radiometric data suchthat the output matches the vector v.

Receive Channel Estimates and Equalizer

The channel model in the cascaded transfer function from transmit arrayto the tag and from the tag to the receive array. Let h_(ij) ^(t) andh_(ij) ^(r) respectively denote quaternion valued impulse response where

$H = \begin{bmatrix}{h_{11}^{t}h_{11}^{r}} & \ldots & {h_{1N}^{t}h_{1M}^{r}} \\\vdots & \ddots & \vdots \\{h_{N\; 1}^{t}h_{1N}^{r}} & \ldots & {h_{NM}^{t}h_{NM}^{r}}\end{bmatrix}$

It is well known

$H = {{U\begin{pmatrix}P^{r} & 0 \\0 & 0\end{pmatrix}}V^{\Delta}}$

where P is a real diagonal matrix and U(^(N×N)) and V^((M×M)) are upperand lower quaternion signals value decomposition in quaternion. Thereare number of approaches to solve for H by zeroforcing or minimum meansquare error nulling. Let us for simplicity denote each filter in thefilter bank as h⁰(z). The prototype digital filter is H⁰(z)H^(notch)(z)where H^(notch) (z) is a notch filter to eliminate any residual DC toenter into one system. Standard multirate symbol processing techniquesfor design of a filter based can be used to arrive at the coefficientsof the impulse response of the filter. The pseudo inverse for H issubsequently used for transmit filter bank.

Alternating Signed Matrices—Transmit Array

Optionally due to practical constraints instead of transmit array inFIG. 7, a configuration for a transmit array such as that in FIG. 7A maybe used.

That is each state of antenna polarization ∈ {−1,1}. Each set ofpolarization is fixed for the query cycle. In this case, it is possibleto use special class of matrices to have mutually orthogonal properties,from one query round to the next.

In case of NLOS and rich scattering environment, it is desirable toconstruct a rich set of observables capturing the reflection of singlebounce rays from the tag to the reader. When only using the preamble forTODA, the short length of the sequence lends itself for applyingHademard or Walsh matrices with property BB^(T)=I. Note in this theoutput is complex value only, since single polarization is used in eachelement.

Assume number of transmit element in the array is 2N, then

$B_{2k} = \begin{bmatrix}B_{{2k} - 1} & B_{{2k} - 1} \\B_{{2k} - 1} & {- B_{{2k} - 1}}\end{bmatrix}$

with

$B_{2} = \begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}$

B_(2k)=H₂⊗2^(N−1), where ⊗ is Kronecker product.

Additional Considerations

In summary consider: the signal s(t) in equation (2) that consists of aspreading sequence p(t) for ranging and data modulation signal such asFM0 or Miller can be represented in quaternion that reflects the linearpolarization of tag in any given position (x_(m),y_(m),z_(m)) in3-dimensional space which will be a linear combination of vertical andhorizontal polarization of a tag which reflects only one polarization inany given direction. The received signal s(t) for the i-th multipath canbe denoted by s_(i)(t)=s(t−τ_(i)) for the i-th multipath. τ_(i) ismultipath delay. A one or two dimensional receiving antenna with Melements with steering vector q(θ_(i),ϕ_(i)) in quaternionrepresentation in direction of (θ_(i),ϕ_(i)) (azimuth and elevationangle) corresponding to the i-th multipath. Channel coefficientsh(θ_(i),ϕ_(i),γ_(i),η_(i)) for the i-th multipath in direction of(θ_(i),ϕ_(i)) (azimuth and elevation angle). (γ_(i),η_(i)) as discussedare angles that relates to amplitude and phase of channel coefficientsfor the i-th multipath. The received vector y for the receiver (one ortwo dimensional array) can be represented as a sum over I for finitenumber of received multipaths of product of h(θ_(i),ϕ_(i),γ_(i),η_(i)),q(θ_(i),ϕ_(i)), and s_(i) (t) plus noise all in quaternions. At thereceiver with antenna array, the receive vector y after channelcoefficient estimation and removing data is correlated with spreadingsequences for possible delays for the each i-th multipath. The output ofcorrelators is a vector call u. As discussed in machine learning for atag at any location (x_(m),y_(m),z_(m)) in 3-dimensional space we obtainu for each possible reader and store it as reference. This can bereferred to as a training phase. In real time location estimation thenwe collect all actual received u vectors from all possible readers andcompare them with all possible stored reference u and choose the closestin Euclidean distance. This will provide the location (range) of thetag.

Although the description above contains many specificities, these shouldnot be construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof the invention. Various other embodiments are possible within itsscope. Accordingly, the scope of the invention should be determined notby the embodiments illustrated, but by the appended claims and theirequivalents.

What is claimed is:
 1. A method for obtaining location information usingan RFID reader system, the method comprising: transmitting a combinedinterrogation and ranging signal from a plurality of antennas, where theranging signal is a pseudorandom signal; receiving a backscatteredreturn signal from an RFID tag at one or more receive antennas;extracting an information signal from the return signal and decoding theinformation signal to obtain RFID tag data; extracting a receivedranging signal from the return signal; and estimating a range to theRFID tag based upon correlation between the ranging signal and thereceived ranging signal.
 2. The method of claim 1, further comprisinggenerating an RFID interrogation signal waveform having a firstfrequency; generating a ranging waveform having a second frequency,where the second frequency is higher than the first frequency; combiningthe RFID interrogation signal and ranging waveform signal into acombined interrogation and ranging signal; splitting the combinedinterrogation and ranging signal to a plurality of transmit pathsthrough a transmit array filter bank and modifying the signal in eachtransmit path using at least one transmit weighting factor, where eachtransmit weighting factor modifies a characteristic of the signal; andtransmitting a first filtered output signal from each of the transmitpaths of the transmit array filter bank using one of a plurality oftransmit antennas in a first interrogation round.
 3. The method of claim2, further comprising adjusting at least one of the at least onetransmit weighting factor based upon the output of the equalizer filterbank and a plurality of the calculated time-of-arrivals of the pluralityof received return signals; and transmitting a second filtered outputsignal from each of the transmit paths of the transmit array filter bankusing one of the plurality of transmit antennas in a secondinterrogation round, where the second filtered output signal is modifiedusing the adjusted at least one transmit weighting factor.
 4. The methodof claim 3, wherein adjusting at least one of the at least one transmitweighting factor comprises applying machine learning to increase thecorrelation of the ranging signals.
 5. The method of claim 3, whereinadjusting at least one of the at least one transmit weighting factorcomprises applying machine learning to increase the read rate of theRFID tag.
 6. The method of claim 1, wherein receiving a backscatteredreturn signal an RFID tag at one or more receive antennas comprisesreceiving a plurality of backscattered return signals from an RFID tagat a plurality of antennas; and the method further comprises combiningthe plurality of received return signals using an equalizer filter bankto produce a combined return signal, and the combining further comprisesmodifying each received return signal using at least one receiveweighting factor, where each receive weighting factor modifies acharacteristic of the signal.
 7. The method of claim 1, whereincombining the RFID interrogation signal and ranging waveform signal intoa combined interrogation and ranging signal comprises adding the twosignals.
 8. The method of claim 1, further comprising adjusting acharacteristic of the transmitted combined interrogation and rangingsignal in a subsequent interrogation round to increase read rate andranging accuracy.
 9. The method of claim 1, further comprisingcalculating the time-of-arrival of each of the plurality of receivedreturn signals.